This invention relates to numerical control (NC) systems, and more particularly to a computer-driven control system for a numerically controlled machine tool.
A typical NC machine tool includes a machine control unit (MCU) and an electric or hydraulic servo-mechanism and drive unit. The machine control unit (MCU) receives a program signal either from a punched paper tape which is fed into a tape reader on the MCU or from an intermediate computer in the case of a direct numerical control (DNC) system or a computer numerical control (CNC) system. The MCU sends a command signal to the machine tool servo-mechanism, and the machine table or cutting tool is moved in response to the command signals from the MCU. Movement of the machine table or cutting tool is monitored by a feedback unit which generates a feedback signal for checking machine operation.
In many NC machining operations, especially those which are relatively simple or only moderately complex, numerical control programs are generated by hand on paper tapes. Hand or off line programming refers to the act of creating a control tape (or control program) by computing all necessary numbers and then using a Flexowriter or an equivalent device to make the punched paper tape of the program. Most smaller NC machine shops utilize hand programming. However, for profiling and other more complex NC machining operations, control tapes are generated with the aid of a computer.
Once an initial control tape has been generated, it must be used to operate the machine to ensure that the part made by the NC machine tool is, in fact, the desired part. This testing procedure is usually referred to as a tape-try out (TTO) cycle. The paper tape is loaded in a tape reader for the machine which interfaces with the MCU. The MCU drives the machine tool so as to produce the part. The part is then checked, and needed corrections are made to the control tape. A new paper tape is then made, tried, and checked. In many instances, 5-10 correction cycles are needed before a hand programmed control tape is proven.
Each tape tryout involves feeding it into the tape reader, and operating the machine tool via the MCU. Of course, to tryout a tape, the machine tool first must be setup for the new tape being tried out. Since correcting the tape may take several hours or even overnight, the machine tool oftentimes must be set up for another job utilizing a proven control tape or must be kept idle while the new tape is being corrected. Since each correction cycle usually requires two machine setups, it is highly desirable to reduce the number of tape tryout cycles needed to prove a new tape. This problem becomes even more acute in short production runs with frequent tooling changes.
Many NC users make use of a computer in generating NC programs. This is done as a batch processing operation and it typically involves a computer terminal connected via telephone lines to a host or central computer having an NC processor. In addition to speeding programming, the computer is also used to check the program. The computer can discover many of the programming and logic errors associated with hand programming, but it cannot discover many dimensional errors which may be present.
Tapes are also checked by computer plotting so as to verify the path of the machine tool cutter before the tape is run through the machine tool. This graphical checkout technique can uncover many errors before the machine tool is taken out of production for an actual tape tryout cycle. While these techniques significantly reduce the number of tape tryouts required to prove a tape, typically one to three tape tryouts still are required. In any event, use of larger computers for the checkout process is expensive.
Paper tape readers themselves have presented significant operational problems. Since the control paper tapes must be physically handled on the shop floor, tapes are often damaged by grease, oil, or other dirt and the tape readers must operate under severe operating environments. Thus, tape readers typically require a substantial amount of maintenance and are a continuing source of system malfunctions.
Direct numerical control (DNC) is an advanced system which bypasses the tape reader. The program data is fed directly into the MCU's in a so-called "behind the tape reader" (BTR) mode. In a DNC system, the control data normally on the paper control tape is stored electrically on disks, for example, in an intermediate program storage memory and is managed by an intermediate computer (usually a minicomputer). A typical prior art DNC system is illustrated in FIG. 1. The intermediate computer transfers data from the intermediate memory to the MCU's and also serves as a remote job processing terminal to a host computer for computer-assisted programming.
More specifically, now referring to FIG. 1, a typical prior art DNC machine tool control system is shown to include an intermediate computer (usually a dedicated minicomputer) which in turn is connected to a host computer which is preferably a large, general purpose digital computer, such as a model 360/168 commercially available from IBM, and which uses a part program language, such as APT (automatic program tools). The intermediate computer is linked to an intermediate program storage memory. A peripherial card reader and a stand-alone line printer are also connected to the intermediate computer. Further, the intermediate computer is connected to a multiplicity of machine control units (MCU). In FIG. 1, the intermediate computer is shown to be connected to eleven MCU's. It will, however, be understood that in many operational DNC systems that many more MCU's are intended to be controlled by the intermediate computer. For example, some commercially available DNC systems are capable of operating or controlling up to 256 MCU's and their respective machine tools. In addition, many DNC systems typically include a communication console (not shown) located on the shop floor adjacent the machine tool so as to permit communication between the shop floor and the host computer. In DNC systems, this conversational programming approach not only permits interaction between the machine operator and the host computer thereby to facilitate NC programming, but also provides a management information system linking all aspects of a manufacturing operation. However, DNC controlled systems are quite expensive and are thus prohibitive for smaller manufacturing companies or for larger companies with only relatively few NC machine tools.
DNC systems have been commercially available for a number of years from various machine tool manufacturers. One such system is described in U.S. Pat. No. 3,668,653, to Donald G. Fair et al, issued June 6, 1972.
Another system has also been developed under the nomenclature "computer numerical control" (CNC). A CNC system is an NC system in which a dedicated stored program computer is used to perform some or all of the NC functions in accordance with control programs stored in the read/write memory of the computer. Even though both DNC and CNC systems utilize computers, the computer supporting a DNC system is used to disseminate manufacturing data to, and to collect data from, many MCU's (see FIG. 1). However, a CNC computer usually supports only one machine tool. Also, the intermediate computer in a DNC system is usually located remotely from the machine tool, while the computer in a CNC system is located on the shop floor.
As noted above, the memory of the CNC dedicated computer (usually a minicomputer) has a read/write memory. This enables the programmer to alter, delete, insert, or otherwise modify the programs in the computer's memory. Also, a library of part programs can be stored in the CNC computer's memory.
In programming a CNC system, the program is typically implemented on a remote general purpose digital computer which is not a part of the CNC system. The program is then punched onto paper tape and this tape is used as an input into the CNC computer at the machine site. It is this paper tape that is modified or corrected during the tape tryout cycles. However, once the tape has been proved, the source deck of the off-line general purpose digital computer is not simultaneously updated. Thus, in a separate operation, the source deck must be updated or the next time the source deck is utilized to generate the program, errors will be present in the program so generated.
A need has long existed for an NC machine tool control system which permits DNC programming and checkout for even single machines and which is of comparatively low cost. The system of this invention accomplishes the purposes of DNC programming for single NC machines while the problems inherent in CNC control systems (e.g., their inability to converse with the remote general purpose computer) have been eliminated.
Reference may also be made to such prior art patents as 3,069,608 and 3,645,298 for further background information in regard to the present invention.
Among the several objects and features of this invention will be noted the provision of a system for the control of a single NC machine in which part programs (i.e., data representation of control tapes) may be stored in memory; The provision of such a system in which the program in a time sharing computer linked to the control system of the present invention can be automatically updated with changes made to the part program made on the shop floor;
The provision of such a system in which only one machine tool would be down in the event of a computer failure;
The provision of such a system in which software incorporated within the time sharing computer is automatically incorporated in the controllers at the site of each NC machine tool;
The provision of such a system which permits real time programming (as opposed to batch processing) when connected to a time sharing host computer thereby to permit corrections to the NC control program to be made at the machine tool site in real time (e.g., within a few minutes) so that the corrected tape may be immediately tried out without having to change the tooling setup of the machine tool;
The provision of such a system which permits data to be moved from one machine tool control system of this invention to another without reprocessing in a host computer thus allowing the machine tools in a manufacturing facility to be more efficiently utilized, and enabling the facility to be readily set up for new manufacturing operations at minimal cost;
The provision of such a system which eliminates problems relating to the reading and handling of punched paper tape and which eliminates tape reader problems;
The provision of such a system which, like DNC systems, substantially reduces the number of tape tryout cycles required to prove a control tape (as compared with prior hand NC programming operations) and, unlike DNC system, is suitable and economical for use with even a single NC machine tool;
The provision of such a system in which the part program may be edited or modified from the shop floor, and in which a revised program can be tried in a matter of minutes thus resulting in program data which can be proved in a matter of hours rather than days with a corresponding savings in time and with a corresponding increase in productivity of the machine tool controlled thereby;
The provision of such a system in which a critical part may be fabricated quickly and in which engineering changes to the part may be readily incorporated in the part program; and
The provision of such a system and method which significantly increases the productivity of the machine tool it controls, which is reliable in operation, which can be adapted for use with prior NC machines, and which is of reasonable cost.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.